Several congenital immunodeficiency diseases are inherited in an X linked recessive fashion, including the most common forms of severe combined immunodeficiency disease (XSCID) and agammaglobulemia (XLA), as well as Wiskott Aldrich syndrome (WA), X linked lymphoproliferative syndrome (XLP), and hyper- IgM syndrome. Genetic counselling, perinatal management of affected infants and gene mapping would be facilitated if carrier detection tests were available, but female carriers of these diseases are completely normal. Preliminary studies indicate that the normal phenotype of female heterozygotes in XSCID, XLA and WA is due to failure of cells in which the mutant gene is on the active X chromosome to enter the mature cell pool. This finding will form the basis for three approaches to diagnosis of the carrier state. The first requires that the two X chromosomes be separated so that the active and inactive X can be identified by restriction fragment length polymorphisms (RFLPs). This will be achieved with somatic cell hybrids made from HPRT-rodent cells and lymphocytes from carriers and their relatives. Because X chromosome RFLPs are abundant this method of carrier detection will succeed in every women, but less laborious techniques will also be pursued based on DNA methylation patterns and on detection of polymorphisms in RNA. The most efficient technique for assaying X inactivation pattern will be used to determine carrier status of at-risk females. This information will make possible regional localization of XSCID by linkage analysis with RFLPs as well as refinement of the map position of XLA, WA, XLP. Improved carrier ascertainment will also aid genetic studies of heterogeneity, new mutation rates and source of new mutations in X-linked immunodeficiencies.